Broad Range Of Strains Research Articles

A modified micromechanical approach to determine flow stress of work materials experiencing complex deformation histories in manufacturing processes

Work materials experience a broad range of strains, strain rates, and temperatures in many manufacturing processes such as machining, forming, etc. Strain rate has an important effect on the yield and flow stress of work materials, especially metals, since at higher strain rates there is less time for thermally activated events; consequently, it is equivalent to a lowering of the temperature of the materials. On the other hand, it is also true that, for high strain rate deformations such as metal cutting, adiabatic plastic flow may produce significant temperature changes in the materials. Flow stress is significantly affected by the strain rate history; hence, mechanical behavior may not be fully described in terms of a mechanical equation of state relating the instantaneous stress, strain, strain rate, and temperature. Based on the concept of dislocation mechanics, a micromechanical approach with the new concept of temperature coefficient has been explored to overcome the model issues such as negative or constant flow stress above the critical temperatures. The flow stresses of aluminum 6061-T6 and titanium Ti-6Al-4V have been predicted, for the first time, using the modified micromechanical model based on the available baseline high strain rates test data. The constitutive model was further modified and extended to predict flow stress below as well as above the critical temperature. The corresponding model predictions were compared with the experimental data, attaining good agreement.

Characterization of a vero cell-adapted virulent strain of enterovirus 71 suitable for use as a vaccine candidate

Enterovirus 71 (EV71) is a neurotrophic virus that causes seasonal morbidity and mortality in children throughout the world with increasing frequency in recent years. Because of the lack of an effective antiviral agent, primary prevention, including the development of effective vaccines, is a top priority in terms of control strategies. Poliovirus vaccine technology, both live attenuated and inactivated, killed virus vaccines, can be adopted for use with EV71 because of their relatedness. In this study, we have characterized a laboratory-adapted EV71 strain, YN3-4a, which exhibits different characteristics from those of its parent isolate, neu, in having a rapid growth rate in Vero cells, a larger plaque size, and a lower LD 50 in newborn mice. The YN3-4a can be produced at a high viral titer of up to 10 10 tissue culture infective dose (TCID 50) when grown in Vero cells, an approved substrate for virus vaccine production. Mouse antiserum raised against YN3-4a can neutralize a broad range of strains of EV71 isolated at different times from a variety of geographic regions. On passage in Vero cells, YN3-4a remained genetically and phenotypically stable. Many of the above-described features, such as high viral yield, strong immunogenicity, broad-based antigenic coverage, and passage stability, are desirable features in a prototype virus for the development of an inactivated viral vaccine.

Modeling the deformation behavior of Hadfield steel single and polycrystals due to twinning and slip

Stress–strain responses of single and polycrystals of Hadfield steel were modeled using a viscoplastic self-consistent approach. A unique hardening formulation was proposed in the constitutive model incorporating length scales associated with spacing between twin lamellae and grain boundaries. TEM observations lend further support to the length scales incorporated into the constitutive model. Many of the experimental findings were made on [ 1 ̄ 11] and [ 1 ̄ 44] crystal orientations deformed in tension, displaying fine twin lamellae at small strains in addition to slip in intra-twin regions. A natural outcome of the model was the small deformation activity inside the twinned regions and higher deformations between the twins. The model utilized dislocation density as a state variable and predicted the stress–strain responses and texture evolution in single crystals accurately over a broad range of strains. The responses of polycrystals with three grain sizes (100, 300, and 1000 μm) were also captured closely with the model in addition to the twin volume fraction evolution with increasing deformation. Based on the simulations, it was possible to explain unequivocally the upward curvature in stress–strain curves in the single crystals and in coarse grained polycrystals of Hadfield steel. Overall, the combined experimental and modeling efforts provide a reliable tool to characterize slip–twin interaction in low stacking fault energy f.c.c. materials.

Material Constitutive Law for Large Strains and Strain Rates

This note comments on a strain rate pattern found in tensile tests. This strain rate pattern readily suggests that the Cowper-Symonds equation relating the static and dynamic flow stresses with the strain rate is not valid for a broad range of strains unless its coefficients change. A modification of the Cowper-Symonds equation is proposed, and good prediction of the dynamic flow stress at any strain and strain rate level was obtained.

Status and Perspectives in Protective Design

Starting with an introduction into the field of hypervelocity impacts, an overview over current research in the area of protection against space debris is given. Trends and strategies to further develop know-how in protection technology are then discussed. One purpose is to demonstrate that improvements in shield efficiency can be expected. To achieve this aim, a strategy is outlined which tries to avoid the adjustment of numerical and material parameters by fits to penetration experiments. Instead, it is suggested to determine material parameters from carefully selected laboratory tests, covering a broad range of strains, strain rates and stress states. Knowledge of the dynamic material behaviour can then be used for the development of new shield concepts by means of numerical simulation.

Direct measurement of isothermal flow stress of metals at elevated temperatures and high strain rates with application to Ta and Ta [sbnd]W alloys

A technique is developed for measuring the flow stress of metals over a broad range of strains, strain rates, and temperatures, in uniaxial compression. It utilizes a recent, enhanced version of the classical (Kolsky) compression split Hopkinson bar [1], in which a sample is subjected to a single stress pulse of a predefined profile, and then recovered without being subjected to any other additional loading. For the present application, the UCSD's split Hopkinson bar is further enhanced by the addition of a new mechanism by means of which the incident and transmission bars of the split Hopkinson construction are moved into a constant-temperature furnace containing the sample, and gently brought into contact with the sample, as the elastic stress pulse reaches and loads the sample. The sample's temperature is measured by thermocouples which also hold the sample in the furnace. Since straining at high strain rates increases the sample's temperature, the sample is allowed to attain the furnace temperature after each controlled incremental loading, and then is reloaded, using the same stress pulse and strain rate. The technique also allows for checking any recoveries that may occur during unloading and reloading. Using several samples of the same material and testing them at the same strain rate and temperature, but different incremental strains, an accurate estimate of the material's isothermal flow stress can be obtained. Additionally, the modified Hopkinson technique allows the direct measurement of the change in the (high strain-rate) flow stress with a change of the strain rate, while the strain and temperature are kept constant, i.e., the strain rate can be increased or decreased during the high strain-rate test. The technique is applied to obtain both quasi-isothermal and adiabatic flow stresses of tantalum (Ta) and a tantalum-tungsten (Ta W) alloy at elevated temperatures. These experimental results show the flow stress of these materials to be controlled by a simple long-range plastic-strain-dependent barrier, and a short-range thermally activated Peierls mechanism. For tantalum, a model which fits the experimental data over strains from a few to over 100%, strain rates from quasi-static to 40 000/s, and temperatures from −200 to 1000°C, is presented and discussed.

Experimental/ computational evaluation of flow stress at high strain rates with application to adiabatic shear banding

Experimental techniques are described and illustrated for direct measurement of temperature, strain-rate, and strain effects on the flow stress of metals over a broad range of strains and strain rates. The approach utilizes: (1) the dynamic recovery Hopkinson bar technique recently developed at UCSD (Nemat-Nasser et al., Proc. R. Soc. London A20, 371–391), in which samples are subjected to a single predefined stress pulse and then recovered without having been subjected to any additional loads; (2) direct measurement of sample temperature by high-speed infra-red detectors; and (3) ability to change the strain rate during the course of experiment at high strain rates. In this manner, constitutive parameters of elasto-viscoplastic flow of metals and metallic alloys are established and used in constitutive models for large-scale computational simulation of high strain-rate phenomena such as adiabatic shear banding. For application to this kind of very high strain-rate events, it becomes necessary to further tune the constitutive parameters through additional coordinated experimental and computational efforts. To this end, Taylor anvil tests are performed, accompanied by high-speed photographic recording of the deformation, and the results are compared with those obtained by finite-element simulations, leading to fine tuning of parameters in the material's flow stress. This procedure is illustrated for tantalum and tantalum/tungsten alloys, as well as for VAR 4340 steel. The simulations are performed using the PRONTO-2D finite-element code, in which an explicit constitutive algorithm, recently proposed by the first two authors and co-workers, has been implemented; Nemat-Nasser (1991, Mech. Mater. 11, 235–249), Nemat-Nasser and Chung (1992, Comput. Meth. Appl. Mech. Eng. 95, 205–219), and Nemat-Nasser and Li (1992, Comput. Struct. 44 (5), 937–963). This algorithm is always stable and incredibly accurate, independently of the time or strain increments. The constitutive model and the constitutive algorithm are used to simulate adiabatic shear bands produced in a hat-shaped specimen under controlled conditions (Beatty et al., 1991, Proc. 12th Army Symp. on Solid Mechanics, pp. 331–345), arriving at excellent correlation with experiments.

Phenotypic Thermal Stability of Rhizobitoxine‐induced Chlorosis and the Nodulation Controlling Gene, rj11

The rj1 gene in soybeans (Glycine max L. Merrill), prevents nodulation with a broad range of strains of Rhizobium japonicum. Strains of R. japonicum that produce rhizobitoxine induced foliar chlorosis can nodulate rj1 plants in sand or vermiculite culture in the greenhouse. This study was undertaken to determine the effects of temperature on the expression of chlorotic symptoms and nodulation of rj1 plants, and to clarify the relationship between nodulation and chlorosis. Tests with plants cultured in Leonard jars in environmental control chambers maintained at 21, 27, and 32 C indicated that temperature modified the nodulation response of Clark rj1 soybeans with the chlorosis inducing strains 61 and 76. Chlorotic symptoms were beast evident at 21 C. Strain 61 produced good nodulation on rj1 plants at 21 C, but exhibited no induction of chlorosis at that temperature on the cultivar ‘Hawkeye.’ Strain 76 produced strong chlorotic symptoms on both the cultivars Hawkeye and ‘Peking’ at 32 C, but only a low level of nodulation with the rj1 plants. This differential effect of temperature on these two processes suggested that, while rhizobitoxine is correlated with both processes, separate control mechanisms with different temperature optima govern their expression.Increasing the temperature from 21 to 32 C mitigated the expression of the ineffective nodulation response of the Rj1 gene with strain 61 in the cultivar Peking. Despite the low level of nodulation (3.6 nodules per plant) of strain 61 with Peking at 32 C, chlorotic symptoms were clearly apparent.